1. Field of the Invention
The present invention relates to a position detecting apparatus using a diffraction grating formed on an object such as a semiconductor device, and more particularly to an apparatus which is used in alignment of exposure equipment for lithography to detect the relative positional relationship between a mask having an original pattern and an object onto which the original pattern is transferred.
The present invention further relates to a position detecting apparatus of the grating interference type, and more particularly to a position detecting apparatus suitable for high-precision aligners used in positioning wafers, masks, etc. in semiconductor manufacture apparatus and so forth.
2. Related Background Art
Recently, projection type exposure equipment (steppers) have widely been employed as an apparatus for transferring fine patterns of semiconductor devices and the like onto semiconductor wafers with high resolution. In the art of this type steppers, there has conventionally been known an apparatus which has an alignment system utilizing the TTR (through-the-reticle) technique as one of positioning between a reticle (synonymous with a mask) and one shot area on a wafer, namely, as the so-called alignment technique, the TTR technique being designed to simultaneously detect an alignment mark formed around a circuit pattern on the reticle and an alignment mark formed around the shot area on the wafer.
Such alignment technique comprises the steps of detecting both the mark on the reticle and the mark on the wafer with high precision, determining a shift or deviation in the relative position between the two marks, and moving the reticle or wafer in a fine manner so as to correct the shift. For the purpose of imaging the pattern of the reticle on the wafer with high resolving power, it is the general current state of projection type exposure equipment that a projection optical system is corrected satisfactorily in chromatic aberration only against an illumination light for exposure (e.g., g line of wavelength 436 nm, i line of wavelength 365 nm, or KrF excimer laser beam of wavelength 248 nm). This means that in an alignment optical system for detecting both the mark on the reticle and the mark on the wafer through the projection optical system, the light used in illuminating the marks is limited to the wavelength which is the same as or very close to that of the exposure light.
A resist layer remains formed on the wafer surface in the exposure step, and the mark on the wafer is detected through the resist layer during alignment. The resist layer has been designed to have the multi-layer resist structure or the like which is high in absorptivity and low in transmissivity for the exposure light, in order to permit patterning with higher resolution. This however raises a problem that since the illumination light is attenuated until reaching the mark on the wafer and the reflected light (such as the regularly reflected light, the scattered light and the diffracted light) from the mark is also attenuated, the mark on the wafer cannot be recognized by the alignment optical system with the sufficient intensity of light, resulting in reduced accuracy of detecting the mark.
When the illumination light for alignment is irradiated to the mark on the wafer to perform alignment, the resist layer in that irradiated area is exposed by its very nature and the mark on the wafer is hence destroyed while passing through various processes after development. This raises another problem, though not essential, that the mark can no longer be used for alignment in exposure of superposing the next layer.
Therefore, on the basis of the TTR technique adopting an alignment system of different wavelengths (i.e., the technique using the illumination light for alignment and the exposure light different in wavelength from each other) which has been disclosed in Japanese Patent Laid-Open No. 63-153820, a method of optically sensing a one-dimentional diffraction grating mark formed on the wafer or reticle and detecting the position of the wafer or reticle from the resulting pitch information with high resolution (on the order of a fraction--one of several tens fractions of the pitch) has been proposed in Japanese Patent Laid-Open 63-283129, for example.
A variety of methods have so far been proposed and practiced in detecting the position of the grating mark. Among them, the above method disclosed in Japanese Patent Laid-Open 63-283129 is directed to irradiate substantially parallel laser beams toward the grating mark from two directions simultaneously for forming a one-dimentional interference fringe, and to determine the position of the grating mark from the interference fringe. Because of using an interference fringe, that method is also called an interference fringe alignment technique.
The interference fringe alignment technique is divided into two methods; a heterodyne method in which a certain frequency difference is given between two laser beams irradiated from two directions, and a homodyne method which gives no frequency difference. In the homodyne method, a still interference fringe is formed in parallel to the grating and the grating (or the object) requires to be finely moved in the pitch direction thereof for detecting the position of the grating mark. The grating position is determined on the basis of the interference fringe as a reference. Meanwhile, in the heterodyne method, the frequency difference between the two laser beams (i.e., the beat frequency) causes the interference fringe to flow in the pitch direction thereof at a high speed corresponding to the beat frequency. Accordingly, the grating position can be determined not on the basis of the interference fringe, but solely on the basis of a time element (phase difference) incidental to high-speed movement of the interference fringe.
As a result of the experiment, it has been found in the above-stated position detecting system utilizing the interference fringe alignment method that the symmetry of incident angles of the two beams irradiating the grating from two directions is very important. If the symmetry of incident angles is unsteadily fluctuated even to a small extent, tele-centricity will be deteriorated and the ratio of a degree of detection error to the measurement resolution will present a great problem, in view of an intrinsic high level of the measurement resolution. The higher the measurement resolution, the more accurate will be the alignment. However, if the ratio of an amount of detection error becomes large, the alignment accuracy will eventually be limited dependent on an extent of the error, thereby to cancel out the merit of using the interference fringe alignment method.
Many factors are envisaged which may affect and fluctuate the symmetry of incident angles of the two beams. Typically, those factors can be divided into one group attributable to the apparatus structure and the other group attributable to the object (grating mark) used in the position detecting process.
The factors attributable to the apparatus structure are as follows. In the exposure equipment and so forth, object lenses, mirrors, etc. of the alignment system are structurally obliged to move corresponding to changes of the mark position. In optical paths extending from the light source to the grating to produce the two beams, there are disposed various optical elements which are all subjected to manufacture errors and assembly errors in greater or lesser degree.
The factors relating to measurement of the object are principally attributable to the symmetry of incident angles. Thus, the partial surface of the grating to be measured is slightly displaced in the direction of the optical axis for each measurement in a region where the two beams intersect. This means, in the case of sequentially measuring a plurality of grating marks on the wafer as practiced in alignment of wafers, that a different sine error is caused for each mark. The error is mainly dependent on flatness of the wafer surface (including warp, curve or the like).
The similar problems have also been encountered in another method of conventional TTR alignment techniques that a laser beam is condensed into a slit shape to relatively scan bar marks or the like on the wafer, thereby for detecting the scattered or diffracted light in an opto-electric manner. However, the resulting effects have hardly been significant because they are small compared with the noise error occurred in detecting the marks and the overall detection accuracy in consideration of waveform distortion and others. Further, in the method of condensing the laser beam into a slit-like spot, the beam cross-section is made narrow or thin in the pupil plane of a projection lens or an alignment object lens in a direction perpendicular to the direction of slit length at the spot on the wafer. Accordingly, the numerical aperture of the beam becomes larger in the direction of detecting the slit-like mark (i.e., in the direction of width of the slit-like spot) compared with the direction perpendicular thereto. Generally, as the numerical aperture of the beam is increased, the diameter (width) of the spot beam can be reduced in proportion to improve the detecting resolution. Corresponding to the increased numerical aperture, however, the width of the beam waist becomes smaller in the direction of the optical axis, eventually resulting in that the stability is affected or lost in the mark detecting process. In an alternative method of condensing the beam into a circle spot, because the wave front of the beam is changed in the direction of the optical axis on both sides of the center of the beam waist, the intrinsic sharp wave-form of an opto-electric signal could not be obtained if the mark is scanned with respect to the beam waist while being displaced in the direction of the optical axis due to effects caused by flatness of the wafer surface, etc.
Meanwhile, it is also required in the TTR alignment system (i.e., the aligner) for implementing the interference fringe alignment method to move both the object lens for alignment and the mirror at the distal end in the direction of the optical axis in afocal relation dependent on changes of the mark position, as disclosed in Japanese Patent Laid-Open No. 57-142612 by way of example. However, Japanese Patent Laid-Open No. 57-142612 can maintain the image conjugate relation of the alignment mark, but cannot maintain the pupil conjugate relation of the alignment system. As a result, various disadvantages occur in each of the light transmitting and receiving systems which constitute the optical system in which a laser beam is passed through the object lens as with the interference fringe alignment method.
Therefore, an alignment optical system arranged in consideration of maintaining the pupil conjugate relation, too, is disclosed in Japanese Patent Laid-Open No. 58-150924 by way of example. This Patent Laid-Open describes a method of moving a part of the optical system for forming an image of an illumination source in the entrance pupil of an object lens for alignment in interlock relation with the object lens when it is moved to change the observing position.
With such arrangement where an optical correction member in a transmitting optical path for the illumination light is moved to maintain the pupil conjugate relation, however, the optical correction member must be interlocked in movement with the object lens and the alignment optical system becomes complicated in the structure. Particularly, in the case of irradiating light beams to the grating mark for measurement from two directions through the object lens to perform the interference fringe alignment, it is very important to retain the symmetry of an intersect angle and incident angles of those light beams. This necessarily requires to take into account the fact that the symmetry conditions are affected or disturbed with movement of the optical correction member. Thus, the structure of the alignment optical system is further complicated.
As mentioned before, projection type exposure equipment, called steppers, have widely been employed in recent years as an apparatus for transferring fine patterns of semiconductor devices and the like onto semiconductor wafers with high resolution. Of late, particularly, it has been required to increase the density of LSI's manufactured by the steppers, and hence transfer still finer patterns onto the wafers. More precise positioning or alignment is essential to cope with such tendency.
Therefore, an apparatus for detecting the position with higher accuracy by the use of the heterodyne interference method is disclosed in Japanese Patent Laid-Open No. 62-261003 by way of example.
In this apparatus, a Zeeman laser which emits a light beam containing both a P-polarized light and an S-polarized light slightly different in frequency from each other by utilizing the Zeeman effect, is employed as a light source for alignment. The light beam from the Zeeman laser is divided by a polarizing beam splitter into the P-polarized light of frequency f.sub.1 and the S-polarized light of frequency f.sub.2. The polarized light beams thus divided are irradiated through respective reflecting mirrors to a grating mark formed on a reticle (mask) from predetermined two directions. The mask is formed with a window at a position adjacent the grating mark, so that a part of the light beams irradiated to the grating mark passes through the window for irradiating a grating mark formed on a wafer from the predetermined two directions. The diffracted lights produced upon the light beams with different frequencies from each other being irradiated to the respective grating marks from two directions are caused to interfere with each other via polarizing plates in the detection system. Two light beat signals resulted from the interferences of the respective diffracted lights are detected by detectors in an opto-electric manner.
Because a relative phase difference between the two signals corresponds to a shift amount between the substrate and the two light beams intersecting at the grating mark, the mark position can be detected with high accuracy by moving the reticle and the wafer relatively such that the phase difference becomes zero with either one of the detected light beat signals as a reference signal.
In order to perform precise alignment with high accuracy by utilizing the interference method that the diffracted lights produced from each alignment grating mark (RM, WM) upon irradiation of two coherent beams thereto are caused to interfere with each other and the intensity of the resulting interference fringe is detected in an opto-electric manner, the two coherent beams require to be irradiated to the grating mark at a predetermined intersect angle with incident angles set equal to each other.
For instance, assuming that the diffracted lights to be detected are given by diffracted lights (D.sub.1, D.sub.2) produced in the vertical (normal) direction to each grating mark (RM, WM) as shown in FIG. 37, an intersect angle .theta. of two light beams (L.sub.1, L.sub.2) irradiating the grating mark (WM, RM) from two directions is uniquely given by the following equation, because it is equal to the sum of incident angles (.alpha., .beta.) of the two light beams irradiating the grating mark (WM, RM) from the two directions: EQU .theta.=sin.sup.-1 (n.sub.1 .lambda./P)-sin.sup.-1 (n.sub.2 .lambda./P)(1)
Here, the wavelength of a laser source is .lambda., the pitch of the grating is P, the order of the diffracted light D.sub.1 produced by the light beam L.sub.1 irradiating the grating at the incident angle .alpha. is n.sub.1 (n.sub.1 &gt;0), and the order of the diffracted light D.sub.2 produced by the light beam L.sub.2 irradiating the grating at the incident angle .beta. is n.sub.2 (n.sub.2 &lt;0).
As one example, supposing that the diffracted lights to be detected are of .+-.1st order lights, the wavelength .lambda. of the light beams irradiating the grating mark (WM, RM) is 600 nm (0.6 .mu.m), and the pitch P of the grating mark (WM, RM) is 10 .mu.m, the intersect angle .theta. is uniquely determined by: ##EQU1##
However, if the two coherent lights are irradiated to the grating mark (WM, RM) at an angle different from the predetermined intersect angle due to adjustment errors of the apparatus or the like, and the diffracted lights produced upon the irradiation are caused to interfere with each other for obtaining the light beat signal by the detector, it becomes difficult to detect the mark position precisely and steadily because of not only breakdown or change in regularity of the light beat signal, but only a decrease in the contrast.
For the reason, the apparatus disclosed in the above-mentioned Japanese Patent Laid-Open No. 62-261003 is arranged to be capable of adjusting the incident angles of the respective light beams with respect to the grating mark, i.e., the intersect angle, by varying inclinations of two reflecting members disposed on the reticle so that the light beams always irradiate each of the grating marks formed on the reticle and the wafer from the predetermined two directions.
But, if the intersect angle is adjusted with the above method--the intersect point of the two light beams is moved in the vertical (normal) direction of the grating mark. To keep the grating mark (WM, RM) positioned at the intersect point of the two light beams, therefore, the stages holding the wafer and the reticle must be moved together following the movement of the intersect point. Such movement of the reticle and the wafer is not preferable for high-accurate alignment in the standpoints of complicating the apparatus structure and increasing the number of factors which may cause mechanical drifts.
Further, in the case of using a pair of grating marks (RM, WM) having a different value of the pitch P, the intersect angle required in this case is changed to a large extent and hence the intersect point of the two light beams must be moved to a large extent in the vertical direction of the grating mark. This is still objectionable to high-accurate alignment.
That point will be explained in detail with reference to FIG. 38.
As illustrated, reflecting members (M.sub.1, M.sub.2) for adjustment of the intersect angle are assumed to be set such that two light beams (L.sub.1, L.sub.2) having the wavelength of 600 nm can be irradiated to each of grating marks (RM, WM) of the 10 .mu.m pitch disposed in a position A at the intersect angle of 6.9.degree., as indicated by solid lines.
In the case of using another pair of grating marks (RM, WM) each having the 10 .mu.m pitch different from the 10 .mu.m pitch of the above grating marks (RM, WM), the intersect angle .theta. required is now given below from the equation (1): ##EQU2##
To cope with this, when the reflecting members for adjustment of the intersect angle is inclined in their posture from M.sub.1, M.sub.2 to M.sub.1 ', M.sub.2 ' so as to change the intersect angle from 6.9.degree. to 13.9.degree., the paths of the light beams (L.sub.1, L.sub.2) are changed as indicated by broken lines. Since the intersect point of the two light beams is thereby moved by a distance of .DELTA.z in the vertical (normal) direction of the grating mark, the reticle and the wafer having the grating marks (RM, WM) are each moved from the position A to a position B.
Supposing now that the distance D bisecting the interval or spacing between two pivot shafts of the reflecting members is 10 mm, for example, the moved amount .DELTA.z of the intersect point is given by: ##EQU3## The consequent large moved amount of the intersect point is not preferable.